This invention relates generally to injection molding, and more particularly to an injection molding nozzle having an integral electrical heating element surrounded by layered ceramic insulation.
Injection molding nozzles having integral spiral electrical heating elements are well known. For instance, the applicant's U.S. Pat. No. 4,238,671 which issued Dec. 9, 1980 shows a helical electrical heating element cast into a conductive material around a high strength corrosion resistant inner core portion. More recently, the applicant's U.S. Pat. No. 4,865,535 which issued Sept. 12, 1989 shows a nozzle in which the heating element has a multiple thickness extending into a tapered nose portion. As shown in both of these patents, it is also well known to provide insulation between the heated nozzle and the surrounding cavity plate by having an insulative air space between them.
With the development of smaller sized injection molding components and the increased demand for more temperature critical materials, the relationship of heating, cooling and insulation in injection molding systems has become even more critical to successful operation. This is particularly true for systems using temperature assisted or thermal gating. An example of thermal gating using cooling is shown in U.S. Pat. No. 4,687,613 to Tsutsumi which issued Aug. 18, 1987. Examples using heating are shown in the applicant's U.S. Pat. No. 4,911,636 which issued Mar. 27, 1990 and U.S. Pat. No. 4,922,082 to Bredt et al. which issued May 1, 1990. The length of cycle time is also a factor which is critical to the successful operation of these systems. Thus, temperature time response as well as location of the heating and cooling relative to the gate is very important. While these previous nozzles are satisfactory for many applications, in addition to thermal conductivity problems, some of them have the disadvantages that they are relatively costly to make and the location of the heating or cooling elements adjacent the gate structurally weakens them.